Interferometric optical techniques are widely used to measure flatness of a surface of an object. For example, to measure the surface profile of a measurement surface, one can use an interferometer to combine a measurement wavefront reflected from the measurement surface with a reference wavefront reflected from a reference surface to form an optical interference pattern. Spatial variations in the intensity profile of the optical interference pattern correspond to phase differences between the combined measurement and reference wavefronts caused by variations in the profile of the measurement surface relative to the reference surface.
One technique for accurate interferometric measurements of surface profiles is phase-shifting interferometry (PSI). With PSI, an optical interference pattern is recorded for each of multiple phase-shifts between the reference and measurement wavefronts to produce a series of optical interference patterns that span, for example, at least a full cycle of optical interference (e.g., from constructive, to destructive, and back to constructive interference). The optical interference patterns define a series of intensity values for each spatial location of the pattern, wherein each series of intensity values has a sinusoidal dependence on the phase-shifts with a phase-offset equal to the phase difference between the combined measurement and reference wavefronts for that spatial location. The phase-offset for each spatial location is extracted from the sinusoidal dependence of the intensity values to provide a profile of the measurement surface relative the reference surface. The algorithms for such phase extraction are generally referred to as phase-shifting algorithms.
Scanning white-light interferometry (SWLI) is another interferometric technique for measuring surface profiles. With SWLI, reference and measurement wavefronts are derived from a common broadband source, and an optical interference pattern is measured as one scans an optical path difference (OPD) between the reference and measurement wavefronts. The optical interference pattern includes local fringes only where the local OPD is within the coherence length of the broadband source. Thus, the surface profile can be determined from the localization of fringes with respect to the OPD scan position. SWLI shares the advantages of accuracy and high resolution with phase-shifting interferometry, while having additional advantages by virtue of the broad emission spectrum of the source light. The broad spectrum makes it possible to profile surfaces accurately and unambiguously with substantially greater roughness and surface departure than is practical with monochromatic interferometers.
In both techniques, the profiling instrument is typically implemented as a microscope having high magnifying power (M>1) to facilitate high accuracy and resolution. However, an instrument that utilizes a large magnifying power profiles a correspondingly small area of the surface during a single measurement (e.g., 2 mm in a typical instrument). In other words, an instrument with a large magnification profiles a small field of view (FOV) and vice versa. To map surfaces which are larger than the FOV, the instrument translates different portions of the surface into the FOV of the instrument and performs multiple measurements.
Although larger fields of view are of great practical interest, instruments capable of imaging larger FOV exhibit decreased light efficiency, and decreased rough-surface interference contrast, which results in reduced resolution and accuracy.